The present invention relates to a method for mixing together gases flowing from two or more different gas sources and for maintaining the mixed gas concentration or composition as stable as possible and for passing such a gas to a gas collection unit, through which a patient's inhalation and exhalation occurs and from which gas collection unit flows gas to a measuring device for analysing the gas respired by a patient. The invention relates also to an apparatus comprised of a gas collection unit provided with a neck opening for placing the gas collection unit around the head of a patient and of a port for delivering to the gas collection unit the gas to be respired by a patient and of a port for passing the gas contained in the gas collection unit to a measuring device for effecting necessary measurements.
The human metabolism can be measured by means of so-called indirect calorimetry, wherein the concentrations and flow rates of respiratory gases can be measured to determine oxygen consumption (VO2) and carbon dioxide output (VCO2). The measuring results can be used to calculate an estimated energy consumption and respiratory quotient (RQ=VCO2/VO2) as well as the estimated amount of carbohydrates and fats consumed by the organism. The combustion of hydrocarbons produces as much carbon dioxide as it consumes oxygen, whereby RQ=1, while the combustion of fats produces a corresponding ratio of about 0.7.
Indirect calorimetry is particularly used in hospital intensive care units for determining the nutrition demand of critically ill patients who are fed intravenously or parenterally or by means of an abdominal tube or enterally. Since a large number of these patients are linked to a respiratory machine or a respirator, the mixing of inhalation gases and the collection of exhalation gases for a measuring operation directly from a patient's respiratory cycle is relatively simple. One device intended for measuring patients linked with a respirator is disclosed in the Finnish Patent No. 78231.
A demand for applying indirect calorimetry also to spontaneously respiring patients has been ever increasing. This concerns particularly patients having e.g. cancer and various metabolic diseases. One gas collection unit intended for metabolic measurements on spontaneously respiring patients has been disclosed in the Finnish Patent No. 78609. The gas collection unit is often referred to as a canopy.
During or after various medical procedures, the critically ill spontaneously respiring patients are fed with excess oxygen, if necessary. Also these types of adult and child patients, who suffer primarily from cardiac and pulmonary malfunction, are preferably subjected to metabolic measurements. The presently available gas collection units, like the one mentioned above, involve problems in metabolic measurements and those become evident at the time it is necessary to feed a patient with air containing more oxygen than ambient air. Thus, the gas collection unit should be completely sealed. When using a gas collection unit to respire air, the oxygen content of which is equal to that of ambient air, a slight leak is not crucial but, whenever the oxygen content in a gas collection unit differs from the ambient air, even a slight leak can result in a major measuring error and falsify e.g. the rates of energy consumption or respiratory quotient.
Furthermore, in a conventional gas collection unit, wherein a patient receives his or her inhalation air and in which a patient's exhalation air arrives, another source of problems is that, upon supplying excess oxygen into a gas flow running towards a gas collection unit, the patient's own respiration tends to alter the inlet flow and, thus, the oxygen content is constantly varying in synchronization with respiration. For purposes of measurements, the oxygen content should be constant.